Abstract
Abstract. A miniaturized infrared (IR) multi-channel sensor system was realized to determine chemical oil degradation, e.g., oxidation, increasing water content. Different artificially aged oil samples (synthetic motor oil, mineral hydraulic oil and ester-based hydraulic fluid) were prepared by oxidative degradation at elevated temperatures or addition of water, and characteristic degradation features in the IR spectrum were detected using FTIR spectroscopy. In addition, the absorption behavior of water contaminated synthetic motor oil was analyzed with increasing temperature. To determine the influence of different degradation effects on the measurement results the sensor system was characterized with the various oil samples. The system uses a reference channel to suppress the effect of decreasing transmission over the entire spectrum caused, e.g., by increasing soot content in the oil or contamination of the optical path.
Highlights
1.1 Chemistry of oil deteriorationTo determine the type and degree of oil degradation different chemical processes have to be taken into consideration, e.g., oxidation, nitration and sulfation, and increased water content
The fresh oil shows a characteristic peak at 1700 cm−1, which is probably due to an additive with a C = O double bond, e.g., an ester
Hydrocarbon-based oils, i.e., a synthetic motor oil and a mineral hydraulic oil, as well as an ester-based hydraulic fluid, were analyzed using FTIR spectroscopy to identify the relevant spectral features caused by oxidation and water contamination
Summary
To determine the type and degree of oil degradation different chemical processes have to be taken into consideration, e.g., oxidation, nitration and sulfation, and increased water content. Different sensors for online determination of various, mostly physical, oil properties, e.g., viscosity, dielectric constant and water content, have been developed These values indicate the chemical status of the oil only indirectly and a change in the sensor signal can be caused by factors independent of the actual oil deterioration, e.g., a refill with slightly different oil with a different base component mixture or other additives. Optical measurement techniques are promising candidates for monitoring the oil quality, especially in the infrared (IR) spectral region, i.e., multi-channel NDIR (non-diffractive infrared) absorption or IR spectroscopy The latter method is used in laboratories, allowing better correlation of online data and lab results. This paper is based on preliminary results presented in Bley et al (2012a) and has been extended with measurements concerning the influence of temperature changes on the spectral transmission and especially reference measurements of acid number and viscosity These were correlated with results of the measurement system for different oil types
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